Lab talk
Aug 15, 2012
ZrO2 nanoparticles boost colour temperature uniformity in white LEDs
Researchers in Taiwan have fabricated white light-emitting diodes (WLEDs) with high luminous efficiency and superior uniformity in angular-dependent correlated colour temperature (CCT). The group of Hao-Chung Kuo and Chien-Chung Lin from the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University (NCTU), achieved superior uniformity of angular CCT in WLEDs by introducing ZrO2 nanoparticles in the remote phosphor structure. The team found that deviations in CCT could be improved by 58% using ZrO2 nanoparticles compared with the conventional remote phosphor structure. Also, the luminous flux of the ZrO2 nanoparticles remote phosphor structure was observed to increase slightly by 2.25% at a driving current of 120 mA.
In recent years, WLEDs have demonstrated great potential to replace general lighting and illumination devices. A popular method of delivering white light in an LED is to combine a blue LED chip with yellow phosphor, but the technique can lead to colour disharmony and low light extraction between air and phosphor.
In the latest work, 300 nm ZrO2 nanoparticles and silicone binder were uniformly mixed together and then sprayed onto the surface of a phosphor layer by a pulsed method. When the CCT was measured at different emission angles, the angular-dependent deviations were seen to improve from 1000K to 420K over a range of –70 to 70 °C. This is due to the extra scattering capability provided by ZrO2 nanoparticles, which can be verified both by haze measurement and FDTD simulation. On the other hand, because the mixture of ZrO2 nanoparticles and silicone provides a refractive index gradient between air and phosphor layers, the light output was also observed to increase in this design.
A full description of the study can be found in the journal Nanotechnology.
About the author
The study was conducted by researchers from the Semiconductor Laser Technology Laboratory at National Chiao Tung University (NCTU) in Hsinchu, Taiwan. Hsin-Chu Chen and Kuo-Ju Chen are PhD candidates in the Institute of Electro-Optical Engineering at NCTU. They performed the processing, characterization and analysis of the ZrO2-type WLEDs. Hsin-Han Tsai and Shih-Hsuan Chien are graduate students in electro-optical engineering at NCTU, and they measured and analysed the optical properties of ZrO2-type WLEDs. Prof. Hao-Chung Kuo and Chien-Chung Lin lead the Semiconductor Laser Technology Laboratory and guided the project. The group specializes in the synthesis and fabrication of quantum dots, nanostructures and nanoparticles for optoelectronic devices, particularly LEDs, WLEDs and solar cells. The teams would like to thank Kismart Corporation and Helio Optoelectronics Corporation for their technical support.
